1. Field of the Invention
This invention relates to a device for mixing powder. Further, the present invention relates to a process for producing a toner for developing electrostatic images in the image forming methods such as electrophotography, electrostatic recording, electrostatic printing and the like.
2. Related Background Art
As the powder mixing device, there have been known such mixers as the vessel rotation type mixer, the vessel fixed type mixer, the fluidized type mixer and the like.
The vessel rotation type mixer rotates a cylindrical or V-shaped vessel as shown in FIG. 5 and FIG. 6. These devices are batchwise and hence continuous treatment is substantially impossible. Further, mixing of powder particles forming a relatively hard agglomerated mass cannot easily effect disintegration. If there is great difference in physical properties in powder starting materials, there is involved a problem that no good final mixed state can be expected. For solving the above problems, there has been made a contrivance to mount a compulsory stirring blade or a baffle in a mixer, but the above problems have not yet been sufficiently solved.
As the vessel fixed type mixer, there are a mixer of the structure in which a stirring screw in which the stirring blade undergoes planetary movement (revolution) within the vessel by rotation of its supporting implement while under rotation (rotation on its own axis) as shown in FIG. 7 or a mixer in which powder is fluidized in a mixing tank by high speed rotation of the blade at the lower part of the mixing tank to effect mixing as shown in FIG. 8.
With the mixer of the construction as shown in FIG. 7, it is difficult to disintegrate an agglomerated mass formed of fine particles.
The device shown in FIG. 8 is a Henschel mixer, and although it is possible to loosen an agglomerated mass to some extent by means of a blade under high speed rotation by the device, but if it is desired to effect sufficient integration, running for a long time is required. In that case, powder generates heat through collision mutually between particles, whereby there is a fear that they may be denatured. With these devices, uniform dispersion is obtained with difficulty, unless an amount is thrown in a certain amount of volume and mixing for a long time of several minutes to several hours is performed. In that case, because the mixing time is long and also the dust concentration is high, there ensues the problem that the particles once dispersed are agglomerated again. Reagglomeration tendency is more marked as the particle size is finer and/or the chargeability of powder is stronger.
Since the mixing device of the system as shown in FIG. 7 and FIG. 8 is batch system, continuous treatment is impossible. Further, it is difficult to perform uniform mixing in all the regions of the mixing vessel.
For example, as the powder, there is a toner for developing the electrostatic image formed by electrophotography.
As the electrophotographic method, there have been known a large number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 42-23910 and 43-24748. Generally speaking, these are methods in which a photoconductive substance is utilized, an electrical latent image is formed on a photosensitive member by various means, subsequently the latent image is developed by use of a toner and the toner image is transferred onto a transfer material such as paper if necessary, followed by fixing by heating, pressure, hot pressure or solvent vapor to obtain a fixed toner image.
The toner to be used in these methods is triboelectrically charged to positive or negative corresponding to the polarity of the electrostatic latent image to be developed.
As the toner to be used in these developing methods, there can be included a pulverized toner obtained by kneading, pulverizing and if necessary, classifying a mixture comprising at least a binder resin and a colorant, a toner obtained by the polymerization method, or a capsule toner.
As the charging method of toner, there may be included (1) the charge injection method in which charges are injected into a toner which is made electroconductive, (2) the dielectric polarization method utilizing dielectric polarization under electrical field, (3) the ion stream charging method in which a shower of charged ions is poured on the particles by such means as corona charger, (4) the frictional charging method in which a toner is rubbed with a material at the position different in triboelectric charging series from the toner. Among them, in the charge injection method, it is difficult to transfer a toner image onto a material to be fixed such as paper from the latent image surface, because the toner is electroconductive. In the dielectric polarization, it is very difficult to produce sufficiently great charges.
On the other hand, according to the charging method by an ion charger, technical difficulty is involved in exposing a toner uniformly to ion stream, whereby it is extremely difficult to control the charging amount with good reproducibility.
The triboelectric charging method uses electrically insulating toner particles, can impart sufficient charging amount to the toner and also has reproducibility, and hence has been presently used widely. However, since the triboelectric charges are in proportion to the frictional work amount, it is difficult to make the frictional work amount of toner particles always at a constant level in the practical development, whereby excess or shortage of charges may occur, or influence from environmental conditions, particularly humidity, may be exerted.
Toner may be attached on the carrier which is in contact with the toner and imparts triboelectric charges to the toner and/or the surface of the sleeve of developing instrument, and through gradual increase of the toner attached, the triboelectric characteristic values of the carrier and the sleeve are caused to be change. As the result, there is also a tendency that deterioration phenomenon of copy image quality occurs when a large number of copies are taken.
As the means for solving this problem, it has been proposed to add fine particulate powdery colloidal silica alone or together with another functional material into a developing agent. For example, there are Japanese Patent Publication No. 54-16219 (corresponding to U.S. Pat. No. 3,720,617) and Japanese Patent Application Laid-open Nos. 55-120041 and 53-81127. Even silica itself has been improved with an aim to control hydrophobicity or chargeability as shown in Japanese Patent Application Laid-open Nos. 58-60754, 58-186751 and 59-200252 (corresponding to U.S. Pat. No. 4,568,625).
However, as the method for adding these, mere addition, or, mixing with stirring blades of a mixer such as Henschel mixer as shown in FIG. 8 or Papenmeier at a circumferential speed of several m/sec. to 40 m/sec. has been generally practiced. In Henschel mixer, through the rotation of the blades fixed on the rotation axis at the central portion, the colored particles and an additive such as silica are dispersed, whereby a part of the additive is attached electrostatically onto the surface of colored particles, and further a part exists under free state to contribute to the flowability of the colored particles. However, according to this method, the circumferential speed is greatly different at the vicinity of the rotary axis portion at the central portion from that of the tip of the stirring blade, and also since there is no blade-like member at the rotary axis portion, the stirring force and dispersing force will differ partially internally of the device to give readily nonuniform dispersed state. For this reason, irregularity occurs in the state of silica attached onto the colored particle surface, and also color particles (toner particles) attached with poorly dispersed silica are formed. Such silica will be readily freed from the colored particles. The freed silica is liable to be consumed by copying to reduce the amount of silica in the developing instrument, thereby causing lowering in the flowability of colored particles or lowering in the image density, and also the freed silica agglomerated may also cause increase of fog.
In a mixer of the structure such as Henschel mixer, mixing is effected batchwise, and hence the dust concentration during mixing is high, and if uniform dispersion is intended to be effected, it will generally take a long time of several minutes to several 10 minutes. For this reason, the particles once dispersed are susceptible to reagglomeration, whereby heat generation is liable to occur by mutual friction of the particles and friction of particles with blades to form a fused product. When the agglomerated body or fused product formed is mixed into the toner as the final product, lowering in the toner quality will be caused to occur.
On the other hand, there has been also known for long time the thought of securing powdery silica onto the surface of colored particles. One method is to add powdery silica together with a binder for the colored particles, colorant, charge controller, etc., melting and kneading the mixture, cooling the kneaded product, followed by pulverization and, if necessary classification, to form a toner. However, when a toner is produced according to this method, silica exists on the toner surface and in the vicinity thereof, and for obtaining sufficient effect, a large amount of silica must be added during melting and kneading. This is not only accompanied with considerable difficulty in production, but also may be a cause in lowering of fixability, which is particularly conspicuous in thermal fixing toner. According to such method, since the ammount of silica existing on the toner surface is small, the improvement of such problems in image quality cannot be said to be sufficient, although some improvement can be seen. As to addition of silica into toner, examples are shown in Japanese Patent Publication No. 44-18995, Japanese Patent Application Laid-open Nos. 51-81623 and 56-1946.
As the means for dispersing silica onto the surface of colored particles, there is a method in which colored particles and silica powder are added, mixed and heated to the softening point or higher to secure the powder onto the surface of the particles, as exemplified by Japanese Patent Application Laid-open Nos. 54-2741 and 57-125943. However, according to this method, there is a danger that fusion of colored particles may be caused to occur.